Solar cell element

ABSTRACT

This solar cell element, which is increased in the conversion efficiency due to improved effect of passivation, includes a semiconductor substrate in which a p-type first semiconductor region and an n-type second semiconductor region are stacked such that the first semiconductor region is located nearmost a first principal surface side and the second semiconductor region is located nearmost a second principal surface side; and a first passivation film containing aluminum oxide and arranged on the first principal surface side of the first semiconductor region. In the inside of the first passivation film of the solar cell element, the first ratio obtained by dividing the aluminum atomic density by the oxygen atomic density is 0.613 or more and less than 0.667 and the second ratio obtained by dividing the sum of the aluminum atomic density and the hydrogen atomic density by the oxygen atomic density is 0.667 or more and less than 0.786.

TECHNICAL FIELD

The present invention relates to a solar cell element.

BACKGROUND ART

In a solar cell element having a silicon substrate, a passivation filmis provided on a surface of the silicon substrate for reducingrecombination of minority carriers, and a technology using, for example,silicon oxide, aluminum oxide, zinc oxide, or indium tin oxide as thematerial of the passivation film has been proposed (e.g., see JapanesePatent Laid-Open No. 2009-164544).

SUMMARY OF INVENTION Problems to be Solved by the Invention

However, the solar cell elements are demanded to have further improvedconversion efficiencies. For example, it has been intended to enhancethe conversion efficiency by prolonging the time (effective lifetime)necessary for recombining minority carriers at an interface through afurther improvement in passivation effect.

It is known that an effective lifetime τ in a structure of a bulksubstrate having passivation films on both surfaces generally has arelationship: (1/τ)=(1/τb)+(2S/W), wherein τb represents the lifetime ofthe bulk substrate, S represents the surface recombination velocity ofminority carriers on the surface of the bulk substrate, and W representsthe thickness of the bulk substrate.

In an actual solar cell element, for example, the effective lifetime τof a p-type bulk substrate in a p-n junction structure is a function ofa lifetime τb and a surface recombination velocity S, is in proportionalto the lifetime τb, and is in inversely proportional to the surfacerecombination velocity S. That is, it is important to reduce the surfacerecombination velocity S for prolonging the effective lifetime τ.

Accordingly, in order to reduce the surface recombination velocity S, apassivation technology using aluminum oxide films has been being paidattention as a technology of enhancing the effect of passivation, and asolar cell element having a coversion efficiency further enhanced by theuse of an aluminum oxide film has been demanded.

Means for Solving the Problems

In order to solve the above-mentioned problems, the solar cell elementaccording to the present invention includes a semiconductor substrate inwhich a p-type first semiconductor region and an n-type secondsemiconductor region are stacked such that the first semiconductorregion is located nearmost a first principal surface side and the secondsemiconductor region is located nearmost a second principal surfaceside; and a first passivation film containing aluminum oxide andarranged on the first principal surface side of the first semiconductorregion. In the inside of the first passivation film of the solar cellelement, the first ratio obtained by dividing the aluminum atomicdensity by the oxygen atomic density is 0.613 or more and less than0.667 and the second ratio obtained by dividing the sum of the aluminumatomic density and the hydrogen atomic density by the oxygen atomicdensity is 0.667 or more and less than 0.786.

Advantageous Effects of Invention

In the solar cell element having the structure described above, theeffective lifetime necessary for recombination of minority carriers isprolonged. That is, the conversion efficiency is further enhancedthrough an improvement in the effect of passivation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view schematically illustrating the external appearanceof the light-receiving surface of a solar cell element according to anembodiment of the present invention.

FIG. 2 is a plan view schematically illustrating the external appearanceof the non-light-receiving surface of a solar cell element according toan embodiment of the present invention.

FIG. 3 is an XZ sectional view along the alternate long and short dashline III-III in FIGS. 1 and 2.

FIG. 4 is an exploded view schematically illustrating a cross section ofa solar cell module according to an embodiment of the present invention.

FIG. 5 is a plan view schematically illustrating the external appearanceof a solar cell module according to an embodiment of the presentinvention.

FIG. 6 is a flow chart for the production of a solar cell elementaccording to an embodiment of the present invention.

FIG. 7 is a plan view illustrating a region at which a thirdsemiconductor region is formed in a solar cell element according to anembodiment of the present invention.

FIG. 8 is a graph showing a relationship between the first ratio and thesecond ratio in samples S1 to S5, A1, and A2.

FIG. 9 is a graph showing a change in the first ratio depending on thedepth from the surface of the first passivation layer in each of samplesS1 to S3.

FIG. 10 is a graph showing a change in the first ratio depending on thedepth from the surface of the first passivation layer in each of samplesS4 to S6.

FIG. 11 is a graph showing a change in the first ratio depending on thedepth from the surface of the first passivation layer in sample S7.

FIG. 12 is a graph showing the effective lifetimes of samples S1 to S7.

FIG. 13 is a graph showing the measurement results by SIMS of sample S9.

FIG. 14 is a graph showing a relationship between the atomic densitiesof hydrogen and carbon and the effective lifetime.

FIG. 15 is a graph showing a relationship between the atomic densitiesof hydrogen and carbon and the effective lifetime.

FIG. 16 is a graph showing the third ratios in samples S2, S3, and S8 toS12.

FIG. 17 is a graph showing the effective lifetimes in samples S2, S3,and S8 to S12.

MODE FOR CARRYING OUT OF THE INVENTION

An embodiment and various modifications of the present invention willnow be described with reference to the drawings. In the drawings, theportions having the same structures and functions are denoted by thesame symbols, and duplicate explanations are omitted in the followingdescription. The drawings are schematic, and the sizes, positionalrelations, and other factors of each structure shown in each drawing canbe appropriately changed. In FIGS. 1 to 5, a right-hand XYZ coordinatesystem where a direction (direction toward the right of the drawing inFIG. 1) orthogonal to the extending direction of the first outputextraction electrode 8 a is defined as the +X direction.

(1) An Embodiment (1-1) Schematic Structure of Solar Cell Element

As shown in FIGS. 1 to 3, the solar cell element 10 includes a firstprincipal surface 10 a, a second principal surface 10 b, and sidesurface 10 c. The second principal surface 10 b is a surface(light-receiving surface) receiving incident light. The first principalsurface 10 a is a surface (non-light-receiving surface) located on theopposite side of the second principal surface 10 b in the solar cellelement 10. The side surface 10 c connects between the first principalsurface 10 a and the second principal surface 10 b. In FIG. 3, thesecond principal surface 10 b is drawn as the upper surface on the +Zside of the solar cell element 10, and the first principal surface 10 ais drawn as the lower surface on the −Z side of the solar cell element10.

The solar cell element 10 also includes a semiconductor substrate 1, afirst passivation layer 5 serving as a first passivation film, a secondpassivation layer 6 serving as a second passivation film, anantireflection layer 7, a first electrode 8, and a second electrode 9.

The passivation effect of the first passivation layer 5 and the secondpassivation layer 6 is the passivation effect (field-effect passivation)by the built-in electric field (formation of an electric field near theinterface by the presence of a passivation layer) and the passivationeffect (chemical passivation) by the termination of dangling bonds ofthe interface. Here, the field-effect passivation means that the effectincreases with an increase in the fixed charge density of thepassivation layer. For example, in p-type silicon, a passivation layerhaving a larger negative fixed charge density is more preferred. Thechemical passivation means that the effect increases with a decrease inthe interface state density.

The semiconductor substrate 1 has a structure in which a firstsemiconductor region 2 and a second semiconductor region 3 are stacked.The first semiconductor region 2 is located nearmost the first principalsurface 1 a (the face on the −Z side in the drawing) side of thesemiconductor substrate 1. The second semiconductor region 3 is locatednearmost the second principal surface 1 b (the face on the +Z side inthe drawing) side of the semiconductor substrate 1. The firstsemiconductor region 2 is a region of the semiconductor showing p-typeconduction, whereas the second semiconductor region 3 is a region of thesemiconductor showing n-type conduction. The first semiconductor region2 and the second semiconductor region 3 form a p-n junction region.

The semiconductor of the first semiconductor region 2 can be crystallinesilicon such as single crystalline silicon or polycrystalline silicon.The first semiconductor region 2 shows p-type conduction by using, forexample, at least one of boron and gallium as the p-type dopant. Thethickness of the first semiconductor region 2 can be, for example, 250μm or less and may be 150 μm or less. The first semiconductor region 2may have any shape. For example, a first semiconductor region 2 having asquare shape in a planar view can be readily produced.

The second semiconductor region 3 is formed in the surface layer of ap-type crystalline silicon substrate (a crystalline silicon substrate)on the second principal surface 1 b side by dispersion of an element asan n-type dopant to the region of the second principal surface 1 b sideof the crystalline silicon substrate. As a result, the region other thanthe second semiconductor region 3 of the crystalline silicon substratecan be the first semiconductor region 2. The n-type dopant may be, forexample, phosphorus.

Furthermore, as shown in FIG. 3, an irregularity portion 11 is providedon the second principal surface 1 b of the semiconductor substrate 1. Aconvex portion of the irregularity portion 11 may have a height of, forexample, 0.1 μm or more and 10 μm or less and may have a width of, forexample, about 1 μm or more and 20 μm or less. A concave portion of theirregularity portion 11 may have, for example, an approximatelyspherical face shape. Herein, the height of a convex portion refers tothe distance from a plane (reference plane) that passes the bottom ofthe concave portion and is parallel to the first principal surface 10 ato the top face of the convex portion in the normal direction of thereference plane. The width of a convex portion refers to the distancebetween the bottoms of adjacent concave portions in the directionparallel to the reference plane.

The first passivation layer 5 as the first passivation film is arrangedon the first principal surface 1 a side of the semiconductor substrate1. That is, the first passivation layer 5 is disposed on the firstsemiconductor region 2 on the first principal surface 1 a side. Thematerial of the first passivation layer 5 may be, for example, aluminumoxide. The presence of the first passivation layer 5 reduces therecombination of minority carriers at the first principal surface 1 a inthe semiconductor substrate 1 by the so-called passivation effect.Consequently, the open circuit voltage and the short circuit current ofthe solar cell element 10 are increased to improve the outputcharacteristics of the solar cell element 10. The first passivationlayer 5 can have an average thickness of, for example, about 3 nm ormore and 100 nm or less.

In this first passivation layer 5, when the aluminum oxide has anegative fixed charge density, the energy band is curved in thedirection of reducing the minority carriers, electrons, in the firstsemiconductor region 2 in the vicinity of the interface with the firstpassivation layer 5. Specifically, in the first semiconductor region,the energy band curves such that the electronic potential increasestowards the interface with the first passivation layer 5. Consequently,the passivation effect by a built-in electric field is enhanced.Furthermore, in this first passivation layer 5, the passivation effectby a built-in electric field and the passivation effect by thetermination of dangling bonds are increased by appropriatelycontrolling, for example, the composition.

The second passivation layer 6 as the second passivation film isarranged on the second principal surface 1 b side of the semiconductorsubstrate 1. That is, the second passivation layer 6 is disposed on thesecond semiconductor region 3 on the second principal surface 1 b side.The presence of the second passivation layer 6 reduces the recombinationof minority carriers in the semiconductor substrate 1 on the secondprincipal surface 1 b side by the passivation effect by the terminationof dangling bonds. Consequently, the open circuit voltage and the shortcircuit current of the solar cell element 10 are increased to improvethe output characteristics of the solar cell element 10. The secondpassivation layer 6 can have an average thickness of, for example, about3 nm or more and 100 nm or less.

Here, when the material of the second passivation layer 6 is aluminumoxide, for example, an antireflection layer 7 having a positiveinterface fixed charge density or a negative interface fixed chargedensity less than that of the aluminum oxide can be disposed on thesecond passivation layer 6. In such a structure, a defect that theenergy band is curved in the direction of increasing the minoritycarriers, holes, is prevented by that in the second semiconductor region3, the second passivation layer 6 has a negative interface fixed chargedensity in the vicinity of the interface with the second passivationlayer 6. As a result, characteristic degradation due to an increase inrecombination of the minority carriers in the semiconductor substrate 1on the second principal surface 1 b side can be inhibited.

The first passivation layer 5 and the second passivation layer 6 areformed by an atomic layer deposition (ALD) method using an organic metalgas containing aluminum, such as trimethylaluminum (TMA) ortriethylaluminum (TEA), as an aluminum supplier and a gas containingoxygen, such as ozone or water, for oxidizing aluminum, as a rawmaterial.

The antireflection layer 7 is a film for enhancing the efficiency ofabsorbing light by the solar cell element 10. The antireflection layer 7is disposed on the second passivation layer 6 on the second principalsurface 10 b side. The material of the antireflection layer 7 can be,for example, silicon nitride or silicon oxide. The thickness of theantireflection layer 7 can be appropriately determined depending on thematerials of the semiconductor substrate 1 and the antireflection layer7. Consequently, the solar cell element 10 achieves a condition in whichlight in a specific wavelength region is hardly reflected. Here, thespecific wavelength region refers to a wavelength region around the peakwavelength of the irradiation intensity of sunlight. When thesemiconductor substrate 1 is a crystalline silicon substrate, theantireflection layer 7 can have a refractive index of, for example,about 1.8 or more and 2.3 or less and can have an average thickness of,for example, about 20 nm or more and 120 nm or less.

The antireflection layer 7 may be disposed on the semiconductorsubstrate 1 on the side surface 10 c side. In such a case, theantireflection layer 7 formed by the ALD method is compact, and,thereby, the formation of fine apertures such as pinholes is notablyreduced also in the side surface 10 c of the semiconductor substrate 1to prevent characteristic degradation due to the occurrence of leakagecurrent.

The third semiconductor region 4 is disposed on the semiconductorsubstrate 1 on the first principal surface 1 a side. The thirdsemiconductor region 4 shows the same p-type conduction as that of thefirst semiconductor region 2. The concentration of a dopant in the thirdsemiconductor region 4 is higher than the concentration of the dopant inthe first semiconductor region 2. That is, the third semiconductorregion 4 is formed by doping the semiconductor substrate 1 with a p-typedopant at a concentration higher than the concentration of the p-typedopant doped to the semiconductor substrate 1 for forming the firstsemiconductor region 2.

The third semiconductor region 4 has a role of reducing recombination ofminority carriers in the region on the first principal surface 1 a sideof the semiconductor substrate 1 by generating a built-in electric fieldon the first principal surface 1 a side of the semiconductor substrate1. Consequently, the presence of the third semiconductor region 4 canfurther enhance the conversion efficiency of the solar cell element 10.The third semiconductor region 4 is formed by, for example, doping thefirst principal surface 1 a side of the semiconductor substrate 1 withan element to be a dopant such as boron or aluminum.

The first electrode 8 is arranged on the first principal surface 10 aside of the semiconductor substrate 1. As shown in FIG. 2, the firstelectrode 8 includes, for example, a plurality of first outputextraction electrodes 8 a extending in the Y direction and a largenumber of first linear collecting electrodes 8 b extending in the Xdirection. The first output extraction electrodes 8 a each at leastpartially intersect with a plurality of the first linear collectingelectrodes 8 b and are electrically connected to the plurality of thefirst collecting electrodes 8 b.

The first collecting electrode 8 b can have a width of, for example,about 50 μm or more and 300 μm or less in the short direction. The firstoutput extraction electrode 8 a can have a width of, for example, about1.3 mm or more and 3 mm or less in the short direction. That is, thefirst collecting electrode 8 b may have any width in the short directionwithin a range that is less than the width in the short direction of thefirst output extraction electrode 8 a. The distance between adjacentfirst collecting electrodes 8 b of the plurality of the first collectingelectrodes 8 b can be about 1.5 mm or more and 3 mm or less. The firstelectrode 8 can have a thickness of, for example, about 10 μm or moreand 40 μm or less. The first electrode 8 can be formed by, for example,applying a conductive paste (silver paste) mainly containing silver ontothe first principal surface 1 a of the semiconductor substrate 1 in adesired pattern by, for example, screen printing and then firing it. Thematerial of the first collecting electrode 8 b may be mainly aluminum,and the material of the first output extraction electrode 8 a may bemainly silver.

The second electrode 9 is arranged on the second principal surface 10 bside of the semiconductor substrate 1. As shown in FIG. 1, the secondelectrode 9 includes, for example, a plurality of second outputextraction electrodes 9 a extending in the Y direction and a largenumber of second linear collecting electrodes 9 b extending in the Xdirection. The second output extraction electrodes 9 a each at leastpartially intersect with a plurality of the second linear collectingelectrodes 9 b and are electrically connected to the second collectingelectrodes 9 b.

The second collecting electrode 9 b can have a width of, for example,about 50 μm or more and 200 μm or less in the short direction. Thesecond output extraction electrode 9 a can have a width of, for example,about 1.3 mm or more and 2.5 mm or less in the short direction. That is,the second collecting electrode 9 b may have any width in the shortdirection within a range that is less than the width in the shortdirection of the second output extraction electrode 9 a. The distancebetween adjacent second collecting electrodes 9 b can be about 1.5 mm ormore and 3 mm or less. The second electrode 9 can have a thickness of,for example, about 10 μm or more and 40 μm or less. The second electrode9 can be formed by, for example, applying a silver paste onto the secondprincipal surface 10 b of the semiconductor substrate 1 in a desiredpattern by, for example, screen printing and then firing it.

(1-2) Passivation Effect

Aluminum oxide usually has a stoichiometric composition Al₂O₃ in whichthe ratio (first ratio) R_(Al/O) of the aluminum atomic density (Al) tothe oxygen atomic density (O) is 2/3. Here, the atomic density refers tothe number of atoms per unit volume and is expressed by the number ofatoms per 1 cm³ (unit: atoms/cm³). When the first ratio R_(Al/O) is lessthan 2/3, specifically, when the first ratio obtained by dividing thealuminum atomic density by the oxygen atomic density is less than 0.667,an Al deficient portion can be present. It is believed that the aluminumoxide forming the passivation layer of this embodiment generates anegative fixed charge density due to the compositional deficiency of Al.In such a case, it is presumed that the aluminum oxide has anonstoichiometric composition and has an amorphous structure similar tothat of γ alumina. This has been confirmed by, for example, transmissionelectron microscopy (TEM) or electron energy-loss spectroscopy (EELS).

In the aluminum oxide of this embodiment, the negative fixed chargedensity is enhanced with an increase in the compositional deficiency ofAl. In order to produce the first passivation layer 5, however, thelower limit of the compositional deficiency of Al as aluminum oxidehaving a nonstoichiometric composition is shown as a composition ofabout Al_(1.9)O_(3.1). Such suppression of Al deficiency can prevent thefilm density (compactness) of aluminum oxide from being excessivelydecreased and hardly deteriorate the film quality. That is, electricleakage hardly occurs, and humidity resistance is hardly decreased.Consequently, the aluminum oxide film can have satisfactory quality and,thereby, can maintain the characteristics and long-term reliability ofthe solar cell element. In this Al_(1.9)O_(3.1), the first ratioR_(Al/O) is about 0.613 (≈1.9/3.1). Aluminum oxide having a first ratioR_(Al/O) of 0.613 or more and less than 0.667 has a large negative fixedcharge density.

If the aluminum oxide contains, for example, hydrogen (H) or CHn (ndenotes a natural number), the dangling bonds of silicon (Si) in thefirst semiconductor region 2 are terminated by H, OH, CHn (n denotes anatural number), O, or other elements at the interface between the firstpassivation layer 5 and the first semiconductor region 2. That is, theeffect of passivation is increased by a reduction in the interface statedensity.

In aluminum oxide having a simple deficiency of Al, electron vacancy isformed in the 2p orbit of O at the Al deficient portion. That is, anacceptor level is generated. In this case, in the first principlescalculation, the fixed charge Q at the Al deficient portion is −3 byreceiving electrons from silicon to which aluminum oxide is conjugateddue to the deficiency of trivalent Al. In contrast, in aluminum oxidecontaining H, the H can be present between the Al deficient portion andthe aluminum oxide lattice. Accordingly, in the aluminum oxide,monovalent H can form a bond (OH bond) with O in the Al deficientportion. On this occasion, the fixed charge Q at the Al deficientportion decreases from −3 to −2. It is, however, presumed that theinstability of the Al deficient portion is eased by the OH bond.Consequently, the stability of aluminum oxide having a nonstoichiometriccomposition containing an Al deficiency is increased. As a result, forexample, the Al deficient portion in the aluminum oxide hardlydisappears even if heat treatment is performed in the formation of theantireflection layer 7, the first electrode 8, and the second electrode9, after the formation of the first passivation layer 5.

Here, when the H atomic density is not lower than the number of the Aldeficient portions in aluminum oxide, H and O bind to each other atalmost all of the Al deficient portions to increase the stability of theAl deficient portions. In such aluminum oxide, the stability of the Aldeficiency protions is increased as long as the ratio of the sum of theAl atomic density and the H atomic density to the O atomic density,i.e., the second ratio, R_((Al+H)/O), obtained by dividing the sum ofthe Al atomic density and the H atomic density by the O atomic densityis 2/3 or more, specifically, 0.667 or more.

In this case, the mechanism of generating a negative fixed charge on thealuminum oxide side is believed, for example, as follows:

Primary reaction Si:Si/Al:O:H→Si.Si/Al:O:+.H

Secondary reaction .H+.H→H:H

Secondary reaction Al:O:CH₃+.H→Al:O:H+.CH₃

In the reaction formulae, the symbol “.” represents an electron, and thesymbol “/” represents an interface. The multiple secondary reactionsmentioned above simultaneously proceed.

In the primary reaction, an electron moves from the Si side to thealuminum oxide side to generate a positive charge on the Si side and anegative charge on the aluminum oxide side.

The positive charge generated on the Si side acts such that the band ofSi is bent upward toward the interface. That is, a potential barrieragainst the electrons as the minority carriers present at the conductionband is generated to inhibit the electrons from flowing into theinterface and disappearing by recombination. In other words, an effectof increasing the effective lifetime of minority carriers is achieved(field-effect passivation).

Meanwhile, the negative charge generated on the aluminum oxide side isfixed, i.e., becomes into a negative fixed charge in the aluminum oxidein the vicinity of the interface. This negative fixed charge in thealuminum oxide is significantly stable and is therefore not lost even inthe process for producing a solar cell element (e.g., a high-temperatureprocess such as firing). That is, the presence of the stable negativefixed charge secures the stability of the positive charge on the Si side(stability of field-effect passivation).

The mechanism described above bends the energy band such that theelectronic energy in the first semiconductor region 2 increases towardthe interface with the first passivation layer 5. Consequently, thepassivation effect by a built-in electric field is enhanced.Furthermore, in this first passivation layer 5, the passivation effectby a built-in electric field is enhanced by appropriately controlling,for example, the composition.

In the aluminum oxide, a higher content of H allows easy binding of H toO in almost all of the Al deficient portions. In order to produce thefirst passivation layer 5, however, the upper limit of the H content isan aluminum oxide having a composition of about (Al+H)_(2.2)O_(2.8) asaluminum oxide having a nonstoichiometric composition. Such an upperlimit can prevent the film density (compactness) of aluminum oxide frombeing excessively decreased to hardly deteriorate the film quality. Thatis, electric leakage hardly occurs, and humidity resistance is hardlydecreased. Consequently, the characteristics and long-term reliabilityof the solar cell element can be maintained.

The second ratio R_((Al+H)/O) of (Al+H)_(2.2)O_(2.8) is about 0.786(≈2.2/2.8). Accordingly, the stability of the Al deficient portions inthe aluminum oxide having a nonstoichiometric composition is enhanced aslong as the second ratio R_((Al+H)/O) is 0.667 or more and less than0.786. That is, the passivation effect by the aluminum oxide is stablygenerated.

The results described above demonstrate that in the inside of the firstpassivation layer 5, passivation effect by aluminum oxide is stablygenerated when the first ratio R_(Al/O) is 0.613 or more and less than0.667 and the second ratio R_((Al+H)/O) is 0.667 or more and less than0.786. As a result, the effective lifetime necessary for recombinationof minority carriers in the first semiconductor region 2 is prolonged.That is, the improvement in the passivation effect further enhances theconversion efficiency of the solar cell element 10.

Here, the inside of the first passivation layer 5 may be, for example,the inner portion of the first passivation layer 5 excluding thevicinities of both principal surfaces in the thickness direction. Thatis, the inside of the first passivation layer 5 does not include thevicinity of the interface with the first semiconductor region 2.Alternatively, the inside of the first passivation layer 5 may be thecentral portion of the first passivation layer 5 in the thicknessdirection. In addition, in the first passivation layer 5, the vicinityof the interface with the first semiconductor region 2 can be, forexample, a region of the first passivation layer 5 defined by thethickness ranging from about 3 nm to 10 nm from the interface. If thethickness of the first passivation layer 5 is 10 nm or less, almost theentire thickness may be recognized as the vicinity of the interface.

When the first ratio R_(Al/O) of the first passivation layer 5 in thevicinity of the interface with the first semiconductor region 2 ishigher than the first ratio R_(Al/O) of the first passivation layer 5 inthe central portion in the thickness direction, the passivation effectby a built-in electric field and the passivation effect by an interfacestate density, i.e., by the termination of dangling bonds of Si at theinterface are enhanced. That is, the effective lifetime of the firstsemiconductor region 2 is prolonged to further enhance the conversionefficiency of the solar cell element 10.

In the first passivation layer 5 containing carbon (C), the passivationeffect is enhanced. For example, in the first passivation layer 5, thesum (total atomic density) A_(H+C) of the atomic density A_(H) of H andthe atomic density A_(C) of C in the vicinity of the interface with thefirst semiconductor region 2 can be higher than the total atomic densityA_(H+C) at the central portion in the thickness direction of the firstpassivation layer 5. In this case, the dangling bonds of Si as thesemiconductor material of the first semiconductor region 2 can beterminated by methyl groups at the interface between the firstpassivation layer 5 and the first semiconductor region 2. Consequently,the passivation effect by the first passivation layer 5 is furtherenhanced. As a result, the effective lifetime of the first semiconductorregion 2 is prolonged to further enhance the conversion efficiency ofthe solar cell element 10.

Furthermore, the effective lifetime of the first semiconductor region 2can be prolonged in proportion to the atomic density A_(H) of H and theatomic density A_(C) of C of the first passivation layer 5 in thecentral portion in the thickness direction and in the vicinity of theinterface with the first semiconductor region 2.

Here, it is presumed that an increase in the atomic density A_(H) of Hstabilizes the Al deficient portions in aluminum oxide and causestermination of the dangling bonds of Si in the interface by, forexample, H, OH, CHn (n denotes a natural number), or O to prolong theeffective lifetime of the first semiconductor region 2, whereas themechanism of prolonging the effective lifetime of the firstsemiconductor region 2 as an increase in the atomic density A_(C) of Cis unclear. It is, however, presumed that when the first passivationlayer 5 is formed by the ALD method using TMA as a raw material, thecontent of CHn (n denotes a natural number) in the passivation layer 5is increased for increasing the atomic density A_(H) of H, which alsoincreases the atomic density A_(C) of C. The CHn may be supplied inother forms such as methane gas, instead of the supply by TMA.

In addition, in the first passivation layer 5, the ratio (the valueobtained by dividing the H atomic density by the C atomic density:hereinafter, referred to as third ratio) R_(H/C) of the H atomic densityto the C atomic density in the vicinity of the interface with the firstsemiconductor region 2 can be higher than the third ratio R_(H/C) in thecentral portion in the thickness direction of the first passivationlayer 5. In this case, since the passivation effect is improved, theeffective lifetime of the first semiconductor region 2 is prolonged tofurther enhance the conversion efficiency of the solar cell element 10.

Here, when the first passivation layer 5 is formed by the ALD method, inthe first passivation layer 5, the third ratio R_(H/C) is higher than 1in the vicinity of the interface with the first semiconductor region 2.Consequently, it is presumed that in the first passivation layer 5, H ismainly in a form of H (or OH) or CHn (n denotes a natural number) in thevicinity of the interface with the first semiconductor region 2. Incontrast, the third ratio R_(H/C) is less than 1 in the region rangingfrom the central portion to the first principal surface 10 a in thethickness direction of the first passivation layer 5. Consequently, itis presumed that H is mainly in a form of H (or OH) and C is mainlybonded to Al or O in the region ranging from the central portion to thefirst principal surface 10 a in the thickness direction of the firstpassivation layer 5.

In this case, it is therefore presumed that the dangling bonds of Si asthe semiconductor material of the first semiconductor region 2 isterminated by, for example, methyl groups at the interface between thefirst passivation layer 5 and the first semiconductor region 2. Such astructure further enhances the passivation effect by the firstpassivation layer 5. As a result, the effective lifetime of the firstsemiconductor region 2 is prolonged to further enhance the conversionefficiency of the solar cell element 10.

(1-3) Solar Cell Module

The solar cell module 100 according to an embodiment includes one ormore solar cell elements 10. For example, the solar cell module 100 mayinclude a plurality of electrically connected solar cell elements 10.Such a solar cell module 100 is formed by, for example, connecting aplurality of solar cell elements 10 in series and in parallel when theelectrical output of each solar cell element 10 is low. For example, apractical electrical output can be extracted by combining a plurality ofsolar cell modules 100. An example of a solar cell module 100 includinga plurality of solar cell elements 10 will now be described.

As shown in FIG. 4, the solar cell module 100 includes laminate of, forexample, a transparent member 104, a front filler 102, a plurality ofsolar cell elements 10, a wiring member 101, a back filler 103, and aback protective material 105. Here, the transparent member 104 is amember for protecting the light-receiving surface of the solar cellmodule 100 receiving sunlight. The transparent member 104 may be anytransparent tabular member, and the material of the transparent member104 is, for example, glass. The front filler 102 and the back filler 103can be, for example, transparent fillers. The materials of the frontfiller 102 and the back filler 103 are, for example, an ethylene-vinylacetate copolymer (EVA). The back protective material 105 is a memberfor protecting the solar cell module 100 from the back. The material ofthe back protective material 105 is, for example, a polyethyleneterephthalate (PET) or polyvinyl fluoride (PVF) resin. The backprotective material 105 may have a single layer structure or a layeredstructure.

The wiring member 101 is a member (connecting member) for electricallyconnecting the plurality of solar cell elements 10. In the solar cellelements 10 adjacent to each other in the Y direction of the solar cellmodule 100, the first electrode 8 of one solar cell element 10 isconnected to the second electrode 9 of the other solar cell element 10with the wiring member 101. Consequently, a plurality of the solar cellelements 10 are electrically connected in series. Here, the wiringmember 101 can have a thickness of, for example, about 0.1 mm or moreand 0.2 mm or less and can have a width of, for example, about 2 mm. Thewiring member 101 can be a member such as copper foil entirely coatedwith solder.

In the solar cell elements 10 electrically connected in series, one endof the electrode of one outermost solar cell element 10 and one end ofthe electrode of the other outermost solar cell element 10 areelectrically connected to a terminal box 107 serving as an outputextraction portion with output extraction wiring 106 respectively.Furthermore, as shown in FIG. 5, the solar cell module 100 may beprovided with a frame 108 (not shown in FIG. 4) for holding the laminatefrom the periphery. The material of the frame 108 is, for example,aluminum having both high corrosion resistance and high strength.

When the material of the front filler 102 is EVA, since EVA containsvinyl acetate, penetration of moisture or water at high temperature maygenerate acetic acid due to hydrolysis with time. In contrast, in thisembodiment, since the antireflection layer 7 is disposed on the secondpassivation layer 6, the damage on the solar cell element 10 caused byacetic acid is reduced. As a result, the reliability of the solar cellmodule 100 is secured for a long time.

When at least one of the materials of the front filler 102 and the backfiller 103 is EVA, an acid acceptor such as magnesium hydroxide orcalcium hydroxide may be added to the EVA. Consequently, the generationof acetic acid from EVA is reduced to enhance the durability of thesolar cell module 100 and reduce the damage on the first passivationlayer 5 and the second passivation layer 6 by acetic acid. As a result,the reliability of the solar cell module 100 is secured for a long time.

(1-4) Method of Producing Solar Cell Element

An example of a process of producing a solar cell element 10 having thestructure described above will now be described. As shown in FIG. 6, thesolar cell element 10 is produced by carrying out the steps SP1 to SP6in order.

In step SP1, a p-type semiconductor substrate 1 is prepared. Here, whenthe semiconductor substrate 1 is a single crystalline silicon substrate,for example, the semiconductor substrate 1 is formed by, for example, afloating zone (FZ) method. When the semiconductor substrate 1 is apolycrystalline silicon substrate, the semiconductor substrate 1 isformed by, for example, casting. An example using a p-typepolycrystalline silicon substrate as the semiconductor substrate 1 willnow be described. First, for example, an ingot of polycrystallinesilicon as a semiconductor material is produced by casting. Then, theingot is cut into semiconductor substrates 1 having a thickness of, forexample, 250 μm or less. The surfaces of the semiconductor substrate 1are then extremely slightly etched with an aqueous solution of, forexample, NaOH, KOH, hydrofluoric acid, or fluonitric acid to remove themechanically damaged or contaminated layers from the cutting planes ofthe semiconductor substrate 1.

In step SP2, an irregularity portion is formed on the second principalsurface 1 b of both the first and second principal surfaces 1 a and 1 bof the semiconductor substrate 1. The irregularity portion is formed,for example, by wet etching using an alkaline solution of NaOH or thelike or an acid solution of fluonitric acid or the like or by dryetching using reactive ion etching (RIE) or the like.

In step SP3, a second semiconductor region 3 showing n-type conductionis formed on the second principal surface 1 b provided with theirregularity portion of the semiconductor substrate 1. The secondsemiconductor region 3 can have a thickness of about 0.2 μm or more and2 μm or less. The second semiconductor region 3 can have a sheetresistance of about 40Ω/□ or more and 200Ω/□ or less. The secondsemiconductor region 3 can be formed by, for example, an applicationthermal diffusion method in which P₂O₅ in a paste form is applied to thesurface of the semiconductor substrate 1 and is then subjected tothermal diffusion or a gas-phase thermal diffusion method in which POCl₃(phosphorus oxychloride) in a gas state is used as a diffusion source.

Here, for example, when the gas-phase thermal diffusion method isemployed, the semiconductor substrate 1 is heat-treated in an atmospherecontaining a diffusion gas such as POCl₃ at a temperature range of about600° C. or more and 800° C. or less. Consequently, phosphorus glass isformed on the second principal surface 1 b of the semiconductorsubstrate 1. The heat treatment time can be, for example, about 5minutes or more and 30 minutes or less. The semiconductor substrate 1 isthen subjected to heat treatment at a high-temperature range of about800° C. or more and 900° C. or less in an atmosphere containing an inertgas such as argon or nitrogen. Consequently, phosphorus diffuses fromthe phosphorus glass to a region on the second principal surface 1 bside of the semiconductor substrate 1 to form the second semiconductorregion 3. The heat treatment time can be, for example, about 10 minutesor more and 40 minutes or less.

During the formation of the second semiconductor region 3, if a secondsemiconductor region 3 is also formed on the first principal surface 1 aside of the semiconductor substrate 1, the second semiconductor region 3formed on the first principal surface 1 a side may be removed byetching. Consequently, the semiconductor region 2 showing p-typeconduction is exposed on the first principal surface 1 a of thesemiconductor substrate 1. For example, the second semiconductor region3 formed on the first principal surface 1 a side is removed by immersingonly the first principal surface 1 a side of the semiconductor substrate1 in a solution of fluonitric acid. Then, the phosphorus glass formed onthe second principal surface 1 b side of the semiconductor substrate 1may be removed by etching. By removing the second semiconductor region 3formed on the first principal surface 1 a side in a state that thephosphorus glass remains on the second principal surface 1 b of thesemiconductor substrate 1, the second semiconductor region 3 on thesecond principal surface 1 b side is prevented from being removed, andfrom being damaged. On this occasion, the second semiconductor region 3formed on the side surface 1 c of the semiconductor substrate 1 may bealso removed in a state that the phosphorus glass remains on the secondprincipal surface 1 b of the semiconductor substrate 1.

Alternatively, the second semiconductor region 3 may be formed by, forexample, a gas-phase thermal diffusion method in a state that adiffusion mask is disposed on the first principal surface 1 a of thesemiconductor substrate 1 and then removing the diffusion mask. In sucha process, no second semiconductor region 3 is formed on the firstprincipal surface 1 a side of the semiconductor substrate 1.Consequently, a process of removing a second semiconductor region 3formed on the first principal surface 1 a side of the semiconductorsubstrate 1 is not necessary.

The method of forming the second semiconductor region 3 is not limitedto those described above. For example, an n-type hydrogenated amorphoussilicon film, a crystalline silicon film including a microcrystallinesilicon film or the like may be formed by a thin-film technology.Furthermore, a silicon region showing i-type conduction may be formedbetween the first semiconductor region 2 and the second semiconductorregion 3.

Subsequently, before the formation of a first passivation layer 5 on thefirst principal surface 1 a of the first semiconductor region 2 andbefore the formation of a second passivation layer 6 on the secondprincipal surface 1 b of the second semiconductor region 3, the surfacesof the first principal surface 1 a and the second principal surface 1 bmay be cleaned with, for example, a combination of nitric acid,hydrofluoric acid, and pure water. Consequently, for example, metalelements such as Na, Cr, Fe, and Cu and natural oxide films, whichadversely affect the electrical characteristics of solar cell elements,are preferably removed as much as possible.

The cleaning treatment can be performed by, for example, cleaning thefirst principal surface 1 a and the second principal surface 1 b withnitric acid at room temperature to about 110° C. for removing the metalelements mentioned above, then rinsing the acid away with pure water,further cleaning the first principal surface 1 a and the secondprincipal surface 1 b with dilute hydrofluoric acid for removing naturaloxide films, and then rinsing the acid away with pure water. Thiscleaning treatment can reduce each of metal elements such as Cr, Fe, andCu to 5×10¹⁰ atoms/cm³ or less.

In the subsequent step SP4, a first passivation layer 5 is formed on thefirst principal surface 1 a of the first semiconductor region 2, and asecond passivation layer 6 is formed on the second principal surface 1 bof the second semiconductor region 3. The first passivation layer 5 andthe second passivation layer 6 can be formed by, for example, an ALDmethod. Consequently, the first passivation layer 5 and the secondpassivation layer 6 are formed on all surfaces of the semiconductorsubstrate 1 at the same time. That is, a passivation layer containing analuminum oxide layer is also formed on the side surface 1 c of thesemiconductor substrate 1.

In the ALD method, the semiconductor substrate 1 provided with thesecond semiconductor region 3 in step SP3 is placed in the chamber of afilm-forming apparatus, and the following processes A to D are repeatedin a state that the semiconductor substrate 1 is heated at a temperaturerange of 100° C. or more and 250° C. or less. Consequently, the firstpassivation layer 5 and the second passivation layer 6 having desiredthicknesses are formed.

[Process A] An Al material is adsorbed onto all surfaces of thesemiconductor substrate 1 by supplying the Al material such as TMAtogether with a carrier gas, such as Ar gas or N₂ gas, onto thesemiconductor substrate 1. On this occasion, the surface of thesemiconductor substrate 1 is desirably terminated with OH groups. Thatis, in a case of a Si substrate, the surface desirably has Si—O—H whenan Al material such as TMA is first supplied. This structure can beformed by, in addition to the above-described cleaning treatment, forexample, conditions for rinsing with pure water in the process oftreating a Si substrate with dilute hydrofluoric acid, and subsequenttreatment with an oxidizing solution such as nitric acid, or subsequenttreatment with ozone. The time for supplying TMA can be, for example,about 15 msec or more and 3000 msec or less. In the process A, thefollowing reaction occurs:

Si—O—H+Al(CH₃)₃→Si—O—Al(CH₃)₂+CH₄↑

[Process B] The Al material in the chamber and also the Al material thathas been physically or chemically adsorbed to the semiconductorsubstrate 1 but has not been chemically adsorbed at an atomic layerlevel are removed by purging the chamber of the film-forming apparatuswith N₂ gas. The time for purging the chamber with N₂ gas can be, forexample, about one second or more and several ten seconds or less.

[Process C] The methyl group as the alkyl group contained in TMA isremoved and is replaced with an OH group by supplying water or anoxidizing agent such as O₃ gas into the chamber of the film-formingapparatus. That is, the following reaction occurs:

Si—O—Al—CH₃+HOH→Si—O—Al—OH+CH₄↑

Here, exactly, “Si—O—Al—CH₃” in the left side should be expressed as“Si—O—Al(CH₃)₂”, but since the notation becomes complicated, theabove-mentioned reaction formula expresses only a reaction of one CH₃.

Consequently, an aluminum oxide atomic layer is formed on thesemiconductor substrate 1. The time for supplying the oxidizing agentinto the chamber may be about 750 msec or more and 1100 msec or less. Inaddition, for example, supply of H together with the oxidizing agentinto the chamber allows the aluminum oxide to easily contain H.

[Process D] The purging of the chamber of the film-forming apparatuswith N₂ gas removes the oxidizing agent in the chamber. At the sametime, for example, the oxidizing agent and other materials that have notcontributed to the reaction for forming aluminum oxide at an atomiclayer level on the semiconductor substrate 1 are also removed. The timeof purging the chamber with N₂ gas can be, for example, about onesecond.

Subsequently, process A is performed again to cause the followingreaction:

Si—O—Al—OH+Al(CH₃)₃→Si—O—Al—O—Al(CH₃)₂+CH₄↑

Furthermore, process B, process C, and process D are performed, and analuminum oxide film having a desired thickness is formed by repeatingprocesses A to D.

Thus, even if the surface of the semiconductor substrate 1 has fineirregularities, an aluminum oxide layer is uniformly formed along theirregularities by forming the first passivation layer 5 and the secondpassivation layer 6 by the ALD method. Consequently, the passivationeffect on the surface of the semiconductor substrate 1 is enhanced.

Subsequently, in step SP5, an antireflection layer 7 is formed on thesecond passivation layer 6 formed on the second principal surface 1 b ofthe semiconductor substrate 1. The antireflection layer 7 can be formedby, for example, a plasma enhanced chemical vapor deposition (PECVD)method, ALD method, evaporation method, or sputtering method. Forexample, when the PECVD method is employed, a gas mixture of SiH₄ gasand NH₃ gas is diluted with N₂ gas in a film-forming apparatus and isformed into plasma by glow discharge decomposition in the chamber, andsilicon nitride is deposited on the second passivation layer 6.Consequently, an antireflection layer 7 containing silicon nitride isformed. The temperature in the chamber during the deposition of siliconnitride can be, for example, about 500° C. An antireflection layer 7having a desired thickness can be formed in a short time by forming theantireflection layer 7 by a method other than the ALD method, such as aPECVD method, evaporation method, or sputtering method. Consequently,the productivity of the solar cell element 10 is enhanced.

Subsequently, in step SP6, a third semiconductor region 4, a firstelectrode 8, and a second electrode 9 are formed.

The methods of forming the third semiconductor region 4 and the firstelectrode 8 will now be described. An aluminum paste containing a glassfrit and aluminum particles is first applied to a predetermined regionof the first passivation layer 5. Subsequently, the components of thealuminum paste burst through the first passivation layer 5 by afire-through method in which heat treatment at a high temperature rangewith a maximum temperature of 600° C. or more and 800° C. or less isperformed to form the third semiconductor region 4 on the firstprincipal surface 1 a side of the semiconductor substrate 1. On thisoccasion, a layer of aluminum is formed on the first principal surface 1a of the third semiconductor region 4. This aluminum layer is used as afirst collecting electrode 8 b which is a part of the first electrode 8.The region where the third semiconductor region 4 is formed can be, asshown in FIG. 7, for example, a region along the broken line 80 definingthe portion for forming the first collecting electrodes 8 b and a partof the first output extraction electrodes 8 a in the first principalsurface 1 a of the semiconductor substrate 1.

The first output extraction electrodes 8 a are produced using, forexample, a silver paste containing a metal powder mainly containingsilver (Ag) and the like, an organic vehicle, and a glass frit.Specifically, a silver paste is applied onto the first passivation layer5 and is then fired to form the first output extraction electrodes 8 a.The maximum temperature of the firing can be, for example, 600° C. ormore and 800° C. or less. In the firing, for example, the temperature israised toward the peak temperature, is maintained at around the peaktemperature for a predetermined time, and is then decreased. The timefor the firing at around the peak temperature can be within severalseconds. The method of applying a silver paste can be, for example, ascreen printing method. After the application of the silver paste, thesilver paste may be dried at a predetermined temperature to evaporatethe solvent in the silver paste. The first output extraction electrodes8 a are brought into contact with the aluminum layer and are therebyelectrically connected to the first collecting electrodes 8 b.

The first collecting electrodes 8 b may be formed after the formation ofthe first output extraction electrodes 8 a. The first output extractionelectrodes 8 a may not be in direct contact with the semiconductorsubstrate 1, and the first passivation layer 5 may be present betweenthe first output extraction electrode 8 a and the semiconductorsubstrate 1. The aluminum layer formed on the third semiconductor region4 may be removed. The first output extraction electrodes 8 a and thefirst collecting electrodes 8 b may be formed with the same silverpaste.

A method of forming a second electrode 9 will now be described. Thesecond electrode 9 is produced using, for example, a silver pastecontaining a metal powder mainly containing Ag and the like, an organicvehicle, and a glass frit. Specifically, a silver paste is applied ontothe second passivation layer 6 of the semiconductor substrate 1 and isthen fired to form the second electrode 9. The maximum temperature ofthe firing can be, for example, 600° C. or more and 800° C. or less. Thetime for the firing can be, for example, within about several seconds atthe firing peak temperature. The method of applying a silver paste canbe, for example, a screen printing method. After the application of thesilver paste, the silver paste may be dried at a predeterminedtemperature to evaporate the solvent in the silver paste. The secondelectrode 9 includes second output extraction electrodes 9 a and secondcollecting electrodes 9 b, which can be formed by a single process atthe same time by employing the screen printing method.

The first electrode 8 and the second electrode 9 may be formed by asingle firing process after the application of the respective pastes.The first electrode 8 and the second electrode 9 exemplified above areformed by printing and firing. But it is not limited to thereto. Thefirst electrode 8 and the second electrode 9 may be formed by any othermethod, for example, a thin film-forming method such as evaporation orsputtering or a plating method.

After the formation of the first passivation layer 5 and the secondpassivation layer 6, the maximum temperature of the heat treatment ineach step is controlled to 800° C. or less, which enhances thepassivation effects of the first passivation layer 5 and the secondpassivation layer 6. For example, in each step after the formation ofthe first passivation layer 5 and the second passivation layer 6, thetime for heat treatment at a temperature range of 300° C. or more and500° C. or less can be, for example, 3 minutes or more and 30 minutes orless.

(1-5) Conclusion of an Embodiment

As described above, in the inside of the first passivation layer 5, thefirst ratio R_(Al/O) can be 0.613 or more and less than 0.667, and thesecond ratio R_((Al+H)/O) can be 0.667 or more and less than 0.786.Consequently, the passivation effect by aluminum oxide of the firstpassivation layer 5 is stably generated. As a result, the effectivelifetime necessary for recombination of minority carriers in the firstsemiconductor region 2 is prolonged. That is, an improvement in thepassivation effect by built-in electric field enhances the conversionefficiency in the solar cell element 10.

In the first passivation layer 5, the first ratio R_(Al/O) in thevicinity of the interface with the first semiconductor region 2 can behigher than that in the central portion in the thickness direction ofthe first passivation layer 5. Consequently, the passivation effect ofthe built-in electric field by the aluminum oxide of the firstpassivation layer 5 and the passivation effect by the interface statedensity, i.e., the termination of the dandling bonds of Si at theinterface are enhanced. That is, the effective lifetime of the firstsemiconductor region 2 is prolonged to further enhance the conversionefficiency of the solar cell element 10.

In the first passivation layer 5, the total atomic density A_(H+C) ofthe atomic density A_(H) of H and the atomic density A_(C) of C in thevicinity of the interface with the first semiconductor region 2 can behigher than that in the central portion in the thickness direction ofthe first passivation layer 5. Consequently, the passivation effect bythe interface state density, i.e., the termination of the dandling bondsof Si at the interface by aluminum oxide of the first passivation layer5 is enhanced. That is, the effective lifetime of the firstsemiconductor region 2 is prolonged to further enhance the conversionefficiency of the solar cell element 10.

In the first passivation layer 5, the third ratio R_(H/C) obtained bydividing the H atomic density by the C atomic density in the vicinity ofthe interface with the first semiconductor region 2 can be higher thanthe third ratio R_(H/C) in the central portion in the thicknessdirection of the first passivation layer 5. Consequently, thepassivation effect by the interface state density, i.e., the terminationof the dandling bonds of Si at the interface by aluminum oxide of thefirst passivation layer 5 is enhanced. That is, the effective lifetimeof the first semiconductor region 2 is prolonged to further enhance theconversion efficiency of the solar cell element 10.

(2) Modification

The present invention is not limited to an embodiment, and variouschanges and improvements are possible within a scope not depart from thegist of the present invention.

For example, in the above-described embodiment, the first passivationlayer 5 is disposed on the non-light-receiving surface side of the solarcell element 10, but is not limited thereto. For example, in thesemiconductor substrate 1, when the n-type semiconductor region isdisposed on the non-light-receiving surface side and the p-typesemiconductor region is disposed on the light-receiving surface side,the first passivation layer 5 can be disposed on the light-receivingsurface side of the solar cell element 10.

The solar cell element 10 may be, for example, a back-contact typehaving a metal-wrap through structure in which the second outputextraction electrodes 9 a are disposed on the first principal surface 10a side.

It should be known that the components of the above-described embodimentand various modifications can be totally or partially combinedappropriately within a range not causing a conflict. In addition, thepassivation layer can be produced by a CVD method, as well as the ALDmethod.

EXAMPLES

Specific examples according to the above-described embodiment will nowbe described.

Preparation of Samples

Single crystalline silicon substrates each having sides of 156 mm and athickness of about 200 μm and showing p-type conduction were prepared assemiconductor substrates 1. Here, an ingot of single crystalline siliconwas formed by an FZ method. On this occasion, the single crystallinesilicon was doped with B so as to show p-type conduction. Subsequently,the single crystalline silicon ingot was sliced into single crystallinesilicon substrates, i.e., semiconductor substrates 1, such that the(100) plane of the silicon single crystal appeared on the firstprincipal surface 1 a and the second principal surface 1 b of eachsemiconductor substrate 1. The surfaces of the semiconductor substrate 1were extremely slightly etched with a ten times diluted aqueous solutionof hydrofluoric acid to remove the mechanically damaged or contaminatedlayers from the cutting planes of the semiconductor substrate 1.

Subsequently, a first passivation layer 5 and a second passivation layer6 mainly containing aluminum oxide were formed by an ALD method on allsurfaces of the semiconductor substrate 1. Here, the semiconductorsubstrate 1 was placed in the chamber of a film-forming apparatus, andthe temperature of the semiconductor substrate 1 was maintained at 175°C., 200° C., or 300° C. The steps A to D were repeated to form the firstpassivation layer 5 and the second passivation layer 6 having desiredthicknesses. Subsequently, in order to investigate the influence of theheat treatment during the formation of the antireflection layer 7, thethird semiconductor region 4, the first electrode 8, and the secondelectrode 9, each semiconductor substrate 1 provided with the firstpassivation layer 5 and the second passivation layer 6 was subjected toeach heat treatment. As a result, samples S1 to S12 were produced.

Furthermore, a first passivation layer 5 and a second passivation layer6 mainly containing aluminum oxide were formed on all surfaces of eachsemiconductor substrate 1 by a CVD method. Here, the semiconductorsubstrate 1 was placed in the chamber of a film-forming apparatus, andthe temperature of the semiconductor substrate 1 was maintained at 150°C. or more and 250° C. to form the first passivation layer 5 and thesecond passivation layer 6 each having a thickness of 30 nm or more and40 nm or less. Subsequently, in order to investigate the influence ofthe heat treatment during the formation of the antireflection layer 7,the third semiconductor region 4, the first electrode 8, and the secondelectrode 9, each semiconductor substrate 1 provided with the firstpassivation layer 5 and the second passivation layer 6 was fired at 810°C. in the firing furnace used for firing the first electrode 8 and thesecond electrode 9 in the atmosphere at a peak temperature of 810° C.for about 1 to 10 seconds. As a result, samples A1 and A2 were produced.

TABLE 1 N₂ gas O₃ gas TMA Substrate Film Effective Flow rate Open timeReplacement Open time Replacement temperature thickness lifetime Sample(sccm) (msec) time (sec) (msec) time (sec) (° C.) (nm) Post-treatment τ(μsec) S1 100 750 15 75 15 175 35 Annealing at 450° C. in N₂ gas 280 S2100 750 15 1000 15 175 40 Annealing at 450° C. in N₂ gas 455 S3 100 75015 75 15 300 30 Annealing at 450° C. in N₂ gas 10 S4 30 900 15 15 12 20030 Annealing at 450° C. in N₂ gas 634 S5 30 900 15 15 12 200 30 Firingat 810° C. in the atmosphere 97 S6 30 900 15 15 12 200 30 None 5 S7 301100 15 15 12 175 30 Firing at 810° C. in the atmosphere 1375 S8 30 110015 15 12 175 30 Firing at 765° C. in the atmosphere 319 S9 30 1100 15 1512 175 30 Firing at 765° C. in the atmosphere 240 S10 100 750 15 75 15175 30 Firing at 765° C. in the atmosphere 358 S11 100 750 15 75 15 17540 Firing at 765° C. in the atmosphere 348 S12 100 750 15 75 15 175 40Firing at 765° C. in the atmosphere 508

The conditions for producing samples S1 to S12 are shown in Table 1.

As shown in Table 1, the temperature of each semiconductor substrate 1of samples S1, S2, and S7 to S12 placed in the chamber of a film-formingapparatus was maintained at about 175° C. The temperature of thesemiconductor substrate 1 of sample S3 was maintained at about 300° C.,and the temperature of each semiconductor substrate 1 of samples S4 toS6 was maintained at about 200° C.

In process A of the ALD method, N₂ gas was used as the carrier gas. Theflow rate of the N₂ gas introduced into the chamber was about 30 sccmfor samples S4 to S9 and was about 100 sccm for samples S1 to S3 and S10to S12. The time (open time) for supplying TMA into the chamber wasabout 15 msec for samples S4 to S9, about 75 msec for samples S1, S3,and S10 to S12, and about 1000 msec for sample S2.

In process B of the ALD method, the time (replacement time) from thecompletion of supply of TMA into the chamber to the start of process Cwas defined as the time for purging the chamber. The replacement timewas about 12 sec for samples S4 to S9 and was about 15 sec for samplesS1 to S3 and S10 to S12. The flow rate of the N₂ gas introduced into thechamber was about 30 sccm for samples S4 to S9 and was about 100 sccmfor samples S1 to S3 and S10 to S12.

In process C of the ALD method, O₃ gas was used as the oxidizing agent.The volume of the O₃ gas introduced into the chamber was about 250 cc atstandard conditions.

The time (open time) for supplying O₃ gas into the chamber was 750 msecfor samples S1 to S3 and S10 to S12, was about 900 msec for samples S4to S6 and was about 1100 msec for samples S7 to S9.

The concentration of the O₃ gas was 300 g/cm³, and the O₃ gas wassupplied once in each process C.

In process D of the ALD method, the time (replacement time) from thecompletion of supply of O₃ gas into the chamber to the start ofsubsequent process was defined as the time for purging the chamber. Thereplacement time was 15 sec for all samples S1 to S12. The flow rate ofthe N₂ gas introduced into the chamber was about 30 sccm for samples S4to S9 and was about 100 sccm for samples S1 to S3 and S10 to S12.

The first passivation layer 5 and the second passivation layer 6 eachhad an average thickness of about 35 nm in sample S1, about 40 nm insamples S2, S11, and S12, and about 30 nm in samples S3 to S10. Theaverage thicknesses were each determined as an average of thicknesses atfive points of each of the first passivation layers 5 and the secondpassivation layers 6 measured with an ellipsometer (SE-400adv,manufactured by SENTECH Co., Ltd.).

The first passivation layer 5 and the second passivation layer 6 wereheat-treated, for samples S1 to S4, by annealing the semiconductorsubstrate 1 at about 450° C. for 20 minutes in N₂ gas; for samples S5and S7, by firing the semiconductor substrate 1 at a peak temperature ofabout 810° C. for about 1 to 10 seconds in the atmosphere; and forsamples S8 to S12, by firing the semiconductor substrate 1 at a peaktemperature of about 765° C. for 1 second or more and 10 seconds or lessin the atmosphere.

Measurement of Composition and Effective Lifetime

Samples S1 to S5, A1, and A2 were used as subjects, and changes in therespective atomic densities A_(Si), A_(Al), A_(O), A_(C), and A_(H) ofSi, Al, O, C, and H depending on the depth from the surface of the firstpassivation layer 5 were measured. Samples S6 and S7 were used assubjects, and changes in the respective atomic densities A_(Al) andA_(O) of Al and O depending on the depth from the surface of the firstpassivation layer 5 were measured. In these measurements, ahigh-resolution analyzer employing rutherford backscatteringspectroscopy (RBS) was used.

Samples S2, S3, S5, and S8 to S12 were used as subjects, and changes inthe respective secondary ion intensities of O, Al, and Si and changes inthe respective atomic densities of H, C, and N depending on the depthfrom the surface of the first passivation layer 5 were measured. Inthese measurements, a secondary ion mass spectrometer (SIMS) was used.

The effective lifetime τ of each of samples S1 to S12, A1, and A2 wasalso measured. In this measurement, a microwave photo conductivity decay(μ-PCD) method using lasers and microwaves was employed.

First Ratio and Second Ratio in the Inside of Passivation Layer

In sample S1, the respective atomic densities A_(Si), A_(Al), A_(O),A_(C), and A_(H) of Si, Al, O, C, and H were substantially constantregardless of the depth in a range inside the first passivation layer 5within a depth range of about 2 to 18 nm from the surface of the firstpassivation layer 5. In also samples S2 to 5, A1, and A2, similarly, therespective atomic densities A_(Si), A_(Al), A_(O), A_(C), and A_(H) ofSi, Al, O, C, and H were substantially constant regardless of the depthin a region in a certain depth range from the surface of the firstpassivation layer 5. It was surmised that in the region in a depth rangeof about 0 to 2 nm from the surface of the first passivation layer 5,the respective atomic densities A_(Si), A_(Al), A_(O), A_(C), and A_(H)of Si, Al, O, C, and H were not substantially constant by influence ofeach element adhered to the surface of the first passivation layer 5.

Table 2 shows the atomic concentration values conveniently convertedfrom the respective atomic densities A_(Al), A₀, and A_(H) of Al, O, andH, the first ratio R_(Al/O), and the second ratio R_((Al+H)/O) in theregion in which the respective atomic densities A_(Si), A_(Al), A_(O),A_(C), and A_(H) of Si, Al, O, C, and H are substantially constant insamples S1 to S5, A1, and A2. Table 2 also shows the effective lifetimesτ of samples S1 to S5, A1, and A2. As described above, the first ratioR_(Al/O) is a value obtained by dividing the Al atomic density A_(Al) bythe O atomic density A_(O), and the second ratio R_((Al+H)/O) is a valueobtained by dividing the sum of the Al atomic density A_(Al) and the Hatomic density A_(H) by the O atomic density A_(O).

TABLE 2 First ratio R_(Al/O) Second ratio R_((Al+H)/O) Effective Al O H(Al atomic density/ (Al + H atomic density/ lifetime Sample (at %) (at%) (at %) O atomic density) O atomic density) τ (μsec) S1 38.0 59.2 2.80.642 0.689 280 S2 37.7 58.9 3.4 0.640 0.698 455 S3 40.3 59.2 0.5 0.6810.689 10 S4 38.3 59.4 2.2 0.645 0.682 634 S5 39.8 59.7 0.5 0.667 0.67597 A1 36.8 58.2 5.1 0.632 0.719 561 A2 36.6 58.7 4.7 0.624 0.704 310

As shown in Table 2, samples S3 and S5 each had a relatively shorteffective lifetime τ, shorter than 100 μsec. In contrast, samples S1,S2, S4, A1, and A2 each had a relatively long effective lifetime τ,longer than 200 μsec.

It was confirmed that in each of samples S1, S2, S4, A1, and A2 havingrelatively long effective lifetimes τ, longer than 200 μsec, the firstratio R_(Al/O) in the inside of the first passivation layer 5 was lessthan 0.667. Specifically, it was confirmed that the first ratiosR_(Al/O) in the inside of the first passivation layers 5 of samples S1,S2, S4, A1, and A2 were 0.642, 0.640, 0.645, 0.632, and 0624,respectively.

In contrast, it was confirmed that in each of samples S3 and S5 havingrelatively short effective lifetimes τ, shorter than 100 μsec, the firstratio R_(Al/O) in the inside of the first passivation layer 5 was 0.667or more. Specifically, it was confirmed that the first ratios R_(Al/O)inside the first passivation layers 5 of samples S3 and S5 were 0.681and 0.667, respectively. Sample S3 was presumed that the first ratioR_(Al/O) was increased due to the high temperature, 300° C., of thesemiconductor substrate 1 during the formation of the first passivationlayer 5. Sample S5 was presumed that the first ratio R_(Al/O) wasincreased due to the generation of H₂O by reaction O and H during thefiring at about 810° C.

It was confirmed that in each of samples S1, S2, S4, A1, and A2 havingrelatively long effective lifetimes τ, longer than 200 μsec, the secondratio R_((Al+H)/O) in the inside of the first passivation layer 5 was0.667 or more. Specifically, it was confirmed that the second ratiosR_((Al+H)/O) in the inside of the first passivation layers 5 of samplesS1, S2, S4, A1, and A2 were 0.689, 0.698, 0.682, 0.719, and 0.704,respectively. In contrast, it was confirmed that the second ratiosR_((Al+H)/O) in the inside of the first passivation layers 5 of samplesS3 and S5 having relatively short effective lifetimes τ, shorter than100 μsec, were 0.689 and 0.675, respectively.

The results above demonstrate that a satisfactory effective lifetime canbe obtained by satisfying the requirements of a first ratio R_(Al/O) of0.613 or more and less than 0.667 and a second ratio R_((Al+H)/O) of0.667 or more and less than 0.786, in the inside of the firstpassivation layer 5. It was presumed that when these requirements aresatisfied, a negative fixed charge is generated by Al deficiency inaluminum oxide, and the Al deficient portion is stabilized by a weakbond between H and O in the Al deficient portion. In addition, it ispresumed that excessive H bonds to dangling bonds of the substratesurface by the heat energy in the subsequent process. That is, it waspresumed that the passivation effect by a built-in electric field due toaluminum oxide of the first passivation layer 5 and the passivationeffect by the interface state density, i.e., the termination of thedandling bonds of Si at the interface are stably generated to furtherenhance the conversion efficiency of the solar cell element.

FIG. 8 is a graph plotting a relationship between the first ratioR_(Al/O) and the second ratio R_((Al+H)/O) in the inside of each firstpassivation layer 5 of samples S1 to S5, A1, and A2. In FIG. 8, thehorizontal axis represents the first ratio R_(Al/O), and the verticalaxis represents the second ratio R_((Al+H)/O). In FIG. 8, therelationship between the first ratio R_(Al/O) and the second ratioR_((Al+H)/O) in the inside of each first passivation layer 5 of samplesS1 to S5, A1, and A2 is shown with seven black circles. The rectangularregion surrounded by a dotted line Ar1 in FIG. 8 satisfies the firstrequirement that the first ratio R_(Al/O) is 0.613 or more and less than0.667 and the second ratio R_((Al+H)/O) is 0.667 or more and less than0.786.

A diagonal line Lc1 of the rectangular region surrounded by the dottedline Ar1 is approximately expressed by an equation:R_((Al+H)/O)=−2×R_(Al/O)+2. As shown in FIG. 8, the seven black circleswere confirmed to be included in the region between straight lines Lmx1and Lmn1, where the line Lmx1 is defined by shifting the diagonal lineLc1 by 0.07 in the direction of increasing the second ratio R_((Al+H)/O)and the line Lmn1 is defined by shifting the diagonal line Lc1 by 0.07in the direction of decreasing the second ratio R_((Al+H)/O). That is,it was confirmed that the second ratio R_((Al+H)/O) is included in arange of −2×first ratio R_(Al/O)+(2−0.07) or more and −2×first ratioR_(Al/O)+(2+0.07) or less. That is, the presence of a second requirementthat the second ratio R_((Al+H)/O) is included in a range of−2×R_(Al/O)+(2−0.07) or more and −2×R_(Al/O)+(2+0.07) or less wasconfirmed. Accordingly, it was demonstrated that a satisfactoryeffective lifetime can be obtained when the aluminum oxide in the firstpassivation layer 5 satisfies, in addition to the first requirement, thesecond requirement.

Change in First Ratio in the Depth Direction of Passivation Layer

FIGS. 9 to 11 are graphs showing changes of the first ratio R_(Al/O)depending on the depth from the surface of each first passivation layer5 of samples S1 to S7. In FIGS. 9 to 11, the horizontal axis representsthe depth from the surface of the first passivation layer 5, and thevertical axis represents the first ratio R_(Al/O). In FIGS. 9 to 11, afirst ratio R_(Al/O) of 2/3 is indicated with an alternate long andshort dash line, as a reference.

In FIG. 9, the curve Ls1 shows a change in the first ratio R_(Al/O) ofsample S1, the curve Ls2 shows a change in the first ratio R_(Al/O) ofsample S2, and the curve Ls3 shows a change in the first ratio R_(Al/O)of sample S3. In FIG. 10, the curve Ls4 shows a change in the firstratio R_(Al/O) of sample S4, the curve Ls5 shows a change in the firstratio R_(Al/O) of sample S5, and the curve Ls6 shows a change in thefirst ratio R_(Al/O) of sample S6. In FIG. 11, the curve Ls7 shows achange in the first ratio R_(Al/O) of sample S7.

FIG. 12 is a bar graph showing the effective lifetimes τ of samples S1to S7. As shown in FIG. 12, the effective lifetimes τ of samples S1, S2,S4, and S7 were relatively long time, 200 μsec or more, whereas theeffective lifetimes τ of samples S3, S5, and S6 were relatively short,less than 100 μsec.

As shown in FIGS. 9 to 11, it was demonstrated that in samples S1, S2,S4, and S7 having relatively long effective lifetimes τ, the first ratioR_(Al/O) notably increases in the first passivation layer 5 from thecentral portion in the thickness direction toward the interface with thefirst semiconductor region 2. In contrast, in samples S3, S5, and S6having relatively short effective lifetimes τ, such an increase in thefirst ratio R_(Al/O) was not observed in the first passivation layer 5from the central portion in the thickness direction toward the interfacewith the first semiconductor region 2.

The results above demonstrated that a satisfactory effective lifetime τcan be obtained by satisfying a requirement that in the firstpassivation layer 5, the first ratio R_(Al/O) in the vicinity of theinterface with the first semiconductor region 2 is higher than the firstratio R_(Al/O) in the central portion in the thickness direction of thefirst passivation layer 5.

Concentration Changes in H and C in the Depth Direction of PassivationLayer

FIG. 13 is a graph showing the measurement results of sample S9 by SIMS.FIG. 13 shows changes in H, C, and N atomic densities and O, Al, and Sisecondary ion intensities depending on the depth from the surface of thefirst passivation layer 5 in sample S9. Specifically, in FIG. 13, thecurve L_(H) drawn with a dark extra thick line shows a change in the Hatomic density; the curve L_(C) drawn with a dark thick line shows achange in the C atomic density; and the curve L_(N) drawn with a darkthin line shows a change in N atomic density. In FIG. 13, the curveL_(O) drawn with a light extra thick line shows a change in the Osecondary ion intensity; the curve L_(Al) drawn with a light thick lineshows a change in the Al secondary ion intensity; and the curve L_(Si)drawn with a light thin line shows a change in the Si secondary ionintensity.

As shown in FIG. 13, the total atomic density A_(H+C) was increased fromthe central portion of the first passivation layer 5 in the thicknessdirection to the interface between the first semiconductor region 2 andthe first passivation layer 5 where the changes in the Si, Al, and Osecondary ion intensities are notable. That is, it was demonstrated thatin the first passivation layer 5, the total atomic density A_(H+C) inthe vicinity of the interface with the first semiconductor region 2 ishigher than the total atomic density A_(H+C) in the central portion inthe thickness direction of the first passivation layer 5.

FIG. 14 is a graph showing a relationship between the H and C atomicdensities and the effective lifetime τ in the first passivation layer 5in the vicinity of the interface with the first semiconductor region 2in samples S2, S3, S5, and S8 to S12. FIG. 15 is a graph showing arelationship between the H and C atomic densities and the effectivelifetime τ in the central portion in the thickness direction of thefirst passivation layer 5 in samples S2, S3, S5, and S8 to S12. In FIGS.14 and 15, the black circles show the H atomic densities, and the whitecircles show the C atomic densities.

As shown in FIGS. 14 and 15, it was confirmed that in the firstpassivation layer 5, the effective lifetime τ increases in proportion tothe H and C atomic densities in both the vicinity of the interface withthe first semiconductor region 2 and the central portion in thethickness direction. In addition, in samples S2 and S8 to S12 havingrelatively long effective lifetimes τ, 200 μsec or more, the H atomicdensity was higher than the C atomic density in the first passivationlayer 5 in the vicinity of the interface with the first semiconductorregion 2, and the atomic densities of H and C were both 5×10²⁰ atoms/cm³or more. In samples S2 and S8 to S12 having relatively long effectivelifetimes τ, 200 μsec or more, the C atomic density was higher than theH atomic density in the central portion in the thickness direction ofthe first passivation layer 5, and the C atomic density was less than1×10²² atoms/cm³.

It was also confirmed that in each first passivation layer 5 of samplesS2 and S8 to S12 having long effective lifetimes τ, the total atomicdensity A_(H+C) in the vicinity of the interface with the firstsemiconductor region 2 is about ten times higher the total atomicdensity A_(H+C) in the central portion in the thickness direction. Itwas therefore confirmed that the passivation effect by the firstpassivation layer 5 is enhanced when the total atomic density A_(H+C) inthe vicinity of the interface with the first semiconductor region 2 ishigher than the total atomic density A_(H+C) in the central portion inthe thickness direction. That is, it is presumed that in this case, theeffective lifetime in the first semiconductor region 2 is prolonged tofurther enhance the conversion efficiency of the solar cell element.

FIG. 16 is a bar graph showing a relationship between the third ratioR_(H/C) in the first passivation layer 5 in the vicinity of theinterface with the first semiconductor region 2 and the third ratioR_(H/C) in the central portion in the thickness direction of the firstpassivation layer 5 with respect to samples S2, S3, S5, and S8 to S12.In FIG. 16, with respect to each of samples S2, S3, S5, and S8 to S12,each bar α in the left shows the third ratio R_(H/C) in the firstpassivation layer 5 in the vicinity of the interface with the firstsemiconductor region 2 and each bar β in the right shows the third ratioR_(H/C) in the central portion in the thickness direction of the firstpassivation layer 5.

As shown in FIG. 16, it was confirmed that in samples S3 and S5, thevalues obtained by dividing the third ratio R_(H/C) in the firstpassivation layer 5 in the vicinity of the interface with the firstsemiconductor region 2 by the third ratio R_(H/C) in the central portionin the thickness direction of the first passivation layer 5 were about 1to 2. In contrast, it was confirmed that in samples S2 and S8 to S12,the values obtained by dividing the third ratio R_(H/C) in the firstpassivation layer 5 in the vicinity of the interface with the firstsemiconductor region 2 by the third ratio R_(H/C) in the central portionin the thickness direction of the first passivation layer 5 were about 3to 10, higher than those in samples S3 and S5.

FIG. 17 is a bar graph showing the effective lifetimes τ of samples S2,S3, S5, and S8 to S12. As shown in FIG. 17, the effective lifetimes τ ofsamples S2 and S8 to S12 were relatively long, 200 μsec or more, whereasthe effective lifetimes τ of samples S3 and S5 were relatively short,less than 100 μsec.

Accordingly, it was demonstrated that a relatively long effectivelifetime τ is obtained when the third ratio R_(H/C) in the firstpassivation layer 5 in the vicinity of the interface with the firstsemiconductor region 2 is three times or more higher than the thirdratio R_(H/C) in the central portion in the thickness direction of thefirst passivation layer 5. It is presumed that in this case, thepassivation effect by aluminum oxide of the first passivation layer 5 isimproved to further enhance the conversion efficiency of the solar cellelement.

In the examples described above, the first passivation layer 5 and thesecond passivation layer 6 were formed by an ALD method on all surfacesof the semiconductor substrate 1 under conditions that the substratetemperature was 175° C. or more. The substrate temperature may be,however, a temperature lower than 175° C., such as 135° C. In addition,the O₃ gas may have a concentration of higher than 300 g/cm³ or may besupplied multiple times for increasing the compositional deficiency ofAl.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   1 semiconductor substrate    -   1 a, 10 a first principal surface    -   1 b, 10 b second principal surface    -   2 first semiconductor region    -   3 second semiconductor region    -   4 third semiconductor region    -   5 first passivation layer    -   6 second passivation layer    -   7 antireflection layer    -   8 first electrode    -   9 second electrode    -   10 solar cell element    -   100 solar cell module

1. A solar cell element comprising: a semiconductor substrate in which ap-type first semiconductor region and an n-type second semiconductorregion are stacked such that the first semiconductor region is locatednearmost a first principal surface side and the second semiconductorregion is located nearmost a second principal surface side; and a firstpassivation film containing aluminum oxide and disposed on the firstsemiconductor region on the first principal surface side, wherein in theinside of the first passivation film, a first ratio obtained by dividingthe aluminum atomic density by the oxygen atomic density is 0.613 ormore and less than 0.667, and a second ratio obtained by dividing thesum of the aluminum atomic density and the hydrogen atomic density bythe oxygen atomic density is 0.667 or more and less than 0.786.
 2. Thesolar cell element according to claim 1, wherein the first ratio of thefirst passivation film in the vicinity of the interface with the firstsemiconductor region is higher than the first ratio in the centralportion in the thickness direction of the first passivation film.
 3. Thesolar cell element according to claim 1, wherein the first passivationfilm contains carbon.
 4. The solar cell element according to claim 1,further comprising: a second passivation film containing aluminum oxideand disposed on the second semiconductor region on the second principalsurface side, wherein the first passivation film and the secondpassivation film each contain carbon.
 5. The solar cell elementaccording to claim 3, wherein a third ratio obtained by dividing thehydrogen atomic density by the carbon atomic density in the firstpassivation film in the vicinity of the interface with the firstsemiconductor region is higher than a third ratio in the central portionin the thickness direction of the first passivation film.
 6. The solarcell element according to claim 5, wherein the hydrogen atomic densityand the carbon atomic density are each 5×10²⁰ atoms/cm³ or more.
 7. Thesolar cell element according to claim 6, wherein the carbon atomicdensity is higher than the hydrogen atomic density in the centralportion in the thickness direction of the first passivation film, andthe carbon atomic density is less than 1×10²² atoms/cm³.